The present invention relates to the field of marine seismic exploration. More particularly, the invention relates to an integrated marine seismic cable system having a flexible cable housing and internal electrical conductor.
In the field of marine seismic exploration, an acoustic signal is generated in a body of water by air guns and other seismic sources. Typically, several air guns are arranged in a selected pattern or arrangement known as a subarray. One or more air gun subarrays are towed behind a vessel to generate an acoustic signal for transmitting energy into subterranean geologic formations.
In a typical marine seismic survey operation, one end of an umbilical seismic cable is connected to an air gun subarray and the other end of the umbilical cable is connected to equipment carried by the seismic vessel. The umbilical cable supplies pressurized air and electrical power to the air guns and also provides contains an electrical conductor for transmitting electricity between the air guns and other equipment. U.S. Pat. No. 5,615,170 to Chelminski (1997) disclosed a representative air gun distribution system having a common high pressure hoseline for distributing compressed air to multiple air guns.
Conventional umbilical cables contain an air hose attached to umbilical cables for transporting pressurized air to the air guns. Multiple insulated electrical conductors are placed around the air hose for carrying electrical power to the air guns and for providing communication between the air guns and the vessel based equipment. An outer shell having armor components and an outer protective jacket is located around the electrical conductors. Each umbilical cable is hundreds of feet long and is normally stored with a rotatable storage reel. The storage reel imparts bending stress on the electrical conductors as the cable is deployed and retrieved from the storage reel.
Conventional umbilical cables combine the electrical conductors as an integral, non-removable part of the umbilical cable during cable fabrication. If one of the electrical conductors is damaged, the damaged electrical conductor cannot be removed from the cable. Efforts to locate and repair the damaged electrical conductor portion are expensive and difficult to perform in the field, and conventional repair requires replacement of the entire cable section with a spare cable section.
Another limitation of conventional umbilical cables is the outside cable diameter. Large cable profiles affect the overall weight and manufacturing cost of the umbilical and the operational costs of storing and towing the umbilical during seismic surveying operations. In a long umbilical cable, a small increase in the cable diameter results in a large increase in the overall weight of the umbilical cable. A larger umbilical cable also results in more drag as the cable is towed behind a marine seismic vessel. This drag increases vessel fuel consumption which increases operational expenses and also causes additional drag forces and the resulting umbilical cable stresses which shorten the usable cable life.
U.S. Pat. No. 5,506,818 to Johnston (1996) disclosed a hose bundle containing an electrical cable disposed within an air hose. The cable was reinforced with multiple, torque balanced armor layers formed with relatively lightweight Kevlar manufactured by the DuPont company. One difficulty of this construction is that Kevlar stretches under strain. This lengthening of the outer covering will transfer stress to the electrical conductor inside the hose bundle, thereby causing parting and other failures of the electrical conductor. To reduce the stretching problems associated with Kevlar, Johnston utilized multiple wrapped layers to spread the total load over multiple Kevlar strands. This created new problems however by increasing the outside dimension of the cable assembly, which increased the weight and drag forces acting on the cable.
The braided Kevlar strands taught by Johnston introduced other problems because the outer surface of the hose bundle is by tag lines in the seismic system. Tag lines comprise secondary lines connecting the hose bundle to offsets or deflectors such as paravanes which pull the hose bundle to a desired offset from the vessel. If the Kevlar strength members are damaged by such contact, parting or other permanent damage to the unbilical and electrical conductor can occur.
As marine seismic exploration evolves, longer marine seismic cables are required and are offset further and further from the vessel centerline. There is a need, therefore, for an improved cable system which reduces drag, which permits maintenance and repair on the vessel, and which increases the useful life of the seismic cables.